Nonemissive displays convey information using contrast differences, which are achieved by varying the reflectance of different frequencies of light; they are thus distinct from traditional emissive displays, which stimulate the eye by emitting light. One type of nonemissive display is an electrophoretic display, which utilizes the phenomenon of electrophoresis to achieve contrast. Electrophoresis refers to movement of charged particles in an applied electric field. When electrophoresis occurs in a liquid, the particles move with a velocity determined primarily by the viscous drag experienced by the particles, their charge (either permanent or induced), the dielectric properties of the liquid, and the magnitude of the applied field.
An electrophoretic display utilizes charged particles of one color suspended in a dielectric liquid medium of a different color (that is, light reflected by the particles) is absorbed by the liquid. The suspension is housed in a cell located between (or partly defined by) a pair of oppositely disposed electrodes, one of which is transparent. When the electrodes are operated to apply a DC or pulsed field across the medium, the particles migrate toward the electrode of opposite sign. The result is a visually observable color change. In particular, when a sufficient number of the particles reach the transparent electrode, their color dominates the display; if the particles are drawn to the other electrode, however, they are obscured by the color of the liquid medium, which dominates instead.
Ideally, the particles maintain a strong uniform charge throughout the lifetime of the device and move as rapidly as possible under the influence of a relatively small electric field. The switching time of suspended particles located between two electrodes is given by ##EQU1## where d is the spacing between electrodes, .eta. is the viscosity of the liquid medium, .epsilon. is its dielectric constant, V is the potential difference between the electrodes, and .zeta. is the zeta potential of the particles. The quantity t represents the time required for the population of particles to migrate from one of the electrodes to the other. Thus, the system is usually selected to minimize t. For example, the spacing between electrodes is as small as is necessary to ensure that the particles are completely obscured following migration away from the transparent electrode.
Useful electrophoretic displays are bistable: their state persists even after the activating electric field is removed. This is generally achieved via residual charge on the electrodes and van der Waals interactions between the particles and the walls of the electrophoretic cell. Unfortunately, the stability of current electrophoretic displays is limited. Although flocculation or settling of particles can be avoided by matching the density of the particles with that of the liquid medium, long-term particle agglomeration remains a problem. That is, cohesive forces among particles may eventually overcome dispersive forces, degrading the appearance and function of the display. For example, particle agglomerations cause visible patterning that detracts from the appearance of the display.
Another drawback of conventional electrophoretic displays is the frequent inability to adequately render a white tonality. For example, in a polychromatic electrophoretic display having ordinary red, green, and blue pigmented pixels, the combined output of such pixels will typically be gray because each is capable of reflecting only part of the incoming light; the additive combination of the reflected light will not provide a true white tonality.